A multicast packet includes a group address that delivers the same packet to more than one destination. If the destination address refers to all nodes on the network, then the packet is referred to as a broadcast packet.
The use of multicast and broadcast packets conserves the bandwidth of a network because only the transmission of a single packet is necessary rather than sending packets individually addressed to each node. This is valuable for wireless networks having limited throughput available.
If a large group of wireless clients, for example, need to receive a particular video stream, then sending multicast packets really pays off. The use of unicast packets to send the stream to each recipient individually would require many separate video streams, resulting in major performance impacts to the wireless network because of the huge amount of bandwidth that would be consumed by the separate streams. With multicast packets, only one stream is necessary.
The use of multicast packets can also facilitate power saving for wireless clients. For example, in an Institute of Electrical and Electronic Engineers (IEEE) 802.11 network, an access point (AP) buffers all multicast frames and sends them immediately following the next DTIM (delivery traffic indication message) if any one client associated with the access point enables power saving mode. With power-save enabled, a client radio will wake up each DTIM beacon and stay awake long enough to receive all of the multicast traffic (occurring after the DTIM beacon).
However, not every DTIM beacon has multicast or broadcast traffic. A zero traffic beacon indication (which can be referred to as “a zero TIM” or “Null Beacon”) is a TIM where the DTIM is zero, indicating that there are no buffered broadcast or, multicast frames, additionally there are no buffered unicast frames and no frame elements requiring action in the regulatory domain (e.g. no DFS Channel Switch Announcement). For devices operating 24/7 (24 hours a day/7 days a week) there may be substantial amount of time (e.g. 60 hours during the week) that the wireless client is not busy transmitting, but still needs to receive beacon frames from an access point. Receiving and processing zero TIM beacon frames can consume substantial amounts of power in such circumstances. Some devices consume eighteen times as much power in receiving as they do when idle [See e.g. SDC-CF10G 802.11g Compact Flash Module with Antenna Connectors available from Summit Data Communications, Inc., 526 South Main Street, Suite 805, Akron, Ohio—http://www.summitdatacom.com/specifications.htm#SDC-PC2CF_PCMCIA_Adapter_for_Compact_Flash_Modules_with_Antenna_Connector s_].